(1) Technical Field of the Invention
This invention relates to nonporous and non-diffusive, polymeric ionic semiconductor materials which can function as novel highly selective permeable membranes driven by ionic depletion gradients and to a method for making such membranes. More particularly, the present invention is directed to materials which have isolated catenated water molecules formed on dispersed and bound hydrogel molecules contained within an inert nonporous nonpermeable matrix, such materials thus having the apparent ability to split water into its constituent ions, and their application as a pervaporative material or in keeping two electrolytes separate while transferring specific ions. The invention is also directed to methods and materials for use in the establishment of reversible chemical-electrical/electrical-chemical energy conversions. Accordingly, the general objects of this invention are to provide novel and improved methods, materials and apparatus of such character.
(2) Description of the Prior Art
Proton selective transport membranes have been of general interest for a number of years since many electrochemical half-cell reactions can be linked with a proton exchange. One of the first known uses of a proton selective membrane was in an early battery known as the Daniell cell. The Daniell cell utilized two separate electrolytes and electrodes, e.g., Zn/ZnSO.sub.4 and CuSO.sub.4 /Cu. A membrane was employed to maintain separation between the metal ions while allowing the free passage of protons. The Daniell cell was not widely used since the best available separator for the cell was an animal membrane with a relatively short life. An attempt was made to substitute fragile, bulky ceramics for the animal membrane in a Daniell cell, but such ceramics were ineffective over extended periods of usage since they allowed eventual mixing of the metal ions through diffusion. The Daniell cell was also not expected to be a secondary or rechargeable battery since in its time it was the only source of electrical power.
Modern proton conductors assume the form of very thin fused glass membranes, such as used in pH sensors, or salts, such as LiN.sub.2 H.sub.5 SO.sub.4, KHF.sub.2, and NH.sub.4 CIO.sub.4 which are not sufficiently conductive at room temperature to be useful in most electrochemical applications. In all known proton conductors, a limited amount of water is known to be present. The water either contributes to or provides an active center or medium for proton transport. Many natural or biological membranes are known to conduct protons at room temperature with fair conductivities, but in general are not suited for commercial application due to their lack of availability or poor chemical and thermal stability.
In U.S. Pat. No. 3,883,784 entitled "Electrical Device With High Dielectric Constant", assigned to the inventor of the present invention, an electrical device having a pair of conductive sheets with a layer of an organic polymeric association product sandwiched between these sheets is disclosed. The patented device functions because of limited proton displacement from bound proton donors. The dried association product preferably comprises polyethylene oxide of a high concentration, i.e., between 60 and 90% by weight of the total, and a polymeric resin preferably a phenolic compound.
U.S. Pat. Nos. 3,390,313 and 3,427,247, respectively entitled "Electromechanical Devices Using Ionic Semiconductors" and "Electroviscous Compositions", disclose a proton conductive coating on silica which operates through proton acceptor and donor sites.
Polymeric permselective membranes do not have specific ion selectivity but rather have variable permeability to specific groups of ions such as anions or cations. Further selectivity may be based upon ionic size, hydration, activity, etc. Selectivity is due to pores of limited size containing isolated charge centers within the pore walls. The charge centers may be furnished by introduction of ion exchange monomers. Some examples of permselective membrane materials are: sulfonated poly- (styrene-divinyl benzene) copolymer, perfluorinated ionomers containing sulfonate and/or carboxylate active sites, or a copolymer of acrylic acid and divinyl benzene. In these polymers, the active charge ion exchange radical appears at various intervals along the polymeric chain resulting in random isolated charges. Accordingly, the distance between the charge sites in such polymers is important since if they are too close swelling of the pores results and if they are too far distant insufficient selectivity is obtained.
Pore size is a basic problem in producing polymeric membranes and most thick film or solid processes involve use of an additive called a pore former, which may be a solvent which evaporates leaving a porous or free volume.
Hydrogels which are characterized by having a high degree of water absorption or the ability to modify water have been employed in electrolytes. Such hydrogels have the ability to modify or immobilize the electrolyte and to form a physical barrier to the migration or diffusion of materials through the structure without significantly lowering the conductivity of the electrolyte. Hydrogels have been used historically as thickeners, film forming agents or as barriers. For example, in drug delivery systems an active drug can be carried in an open immobilized structure of electrolyte and hydrogel. The rate of diffusion of the drug through the structure is controlled by the characteristics of the selected hydrogel. The porosity and hydrophilicity of hydrogels can be decreased by cross-linking the hydrogel or by copolymerizing two different hydrogels. As another example, hydrogels have been used in batteries to provide a barrier which will allow the diffusion of ions, absorb the electrolyte, provide electronic separation and keep the solid particles or constituents separate. The most widely used hydrogel materials have been starches, cellulosics, and natural gums--all of which absorb many times their own weight in water or electrolyte and form gels. These hydrogels also have a high diffusion rate which is important in most applications to single electrolyte systems.
Polymeric "nonporous" membranes are typically thin films with diffusion through the free volume offered by the amorphous phases of the physical structure of long chain polymers. Cellulose and its derivatives are examples of materials which, when in film form, exhibit such behavior. Aromatic polyamide-imides, chemically modified polysulfones, and ethylene oxide grafted "Nylon-6" are examples of noncellulosic membranes.
Permselective membranes have been used to replace anions or cations such as in the sweetening of citrus juice. Typically, a sweetening process uses two anion selective membranes separating the juice from two alkaline electrolytes. A passage of current through all three chambers causes hydroxyl anions to pass from one alkaline electrolyte into the juice to neutralize the acid hydrogen cation while the citrate anions are passed into the other alkaline electrolyte forming a salt.
Certain biological materials such as proteins are known to be semiconductors with high activation energies inversely proportional to absorbed water. These biopolymers have a relatively low ionic conduction, i.e., ionic conduction proportional to water absorption.